Vasovagal syncope is a condition marked by a sudden drop in heart rate and blood pressure, resulting in fainting. It is not only unpleasant for a patient, but potentially dangerous, as fainting may lead to injuries from falls. U.S. Pat. No. 5,284,491, issued to Sutton et al. (the '491 patent) discloses a cardiac pacemaker specifically adapted to treat patients suffering from vasovagal syncope. In particular, according to the '491 patent the pacemaker detects when the patient's heart rate drops below a lower “hysteresis” rate and determines whether the average rate of decrease in the patient's heart rate, over a defined number of heartbeats or a defined time interval prior to reaching the “hysteresis” rate, is greater than a preset value. If so, the pacemaker's rate is set equal to the “hysteresis” rate and thereafter increased to an “intermediate” rate substantially higher than the “hysteresis” rate. The pacemaker's rate remains at the “intermediate” rate for a preset time period and thereafter gradually declines to a lower pacing rate.
Various improvements have been proposed for detecting a heart rate drop and responding to a rate drop with interventional pacing. In U.S. Pat. No. 5,441,525, issued to Shelton, et al., incorporated herein by reference in its entirety, a stability and intervention procedure is proposed in which transient drops in rate are ignored and pacing at a predetermined high rate occurs if the drops are stable. A sleep disable feature disables the vasovagal syncope detection and therapy features during the patient's sleeping hours to reduce or eliminate false positive responses. In U.S. Pat. No. 5,501,701, issued to Markowitz et al., incorporated herein by reference in its entirety, a method is disclosed in which pacing pulses are provided at an increased rate in response to a rapid drop detected when the heart rate falls from a persistent rate above a first threshold rate to a stable rate below a second threshold rate. If spontaneous depolarizations are detected while pacing at the increased rate, pacing at the increased rate is terminated.
In U.S. Pat. No. 5,676,686 issued to Jensen, et al., incorporated herein by reference in its entirety, a rapid drop in heart rate may be detected in response to a drop of greater than a defined amount from the highest detected heart rate over a preceding time period. Alternatively, an unbroken sequence of pacing pulses delivered at a base pacing rate may trigger pacing at an increased pacing rate. The latter rate drop detection criterion is advantageous in patients that experience unpredictable rate drop episodes. As implemented in the commercially available Medtronic Kappa™ 700 pacemaker, a rate drop response provides pacing at an elevated rate for a brief period of time following a drop in heart rate which meets programmed rate drop detection criteria. To detect rate drop episodes in patients having episodes that are predictable in drop size and suddenness, a drop rate, drop size and detection window can be programmed to provide relatively specific rate drop detection. In patients that experience unpredictable rate drop episodes, more general rate drop detection criteria based on lower rate pacing for a predetermined number of consecutive detection beats may be programmed. In patients having both predictable and unpredictable rate drop episodes, both the lower rate criterion and the rate drop size and suddenness criteria and can be enabled to detect a rate drop.
Generally, a rate drop response delivers higher rate pacing for a predetermined interval of time after which the pacing rate is gradually reduced back to the programmed lower rate. If rate drop detection criteria includes a the lower rate criterion and no intervening intrinsic cardiac activity is sensed as the pacing rate is reduced from a high rate back to the lower rate, the rate drop response will be re-triggered after pacing returns to the lower rate even though an intrinsic rate drop has not actually occurred. If the intrinsic heart rate remains below the programmed lower rate, a cyclical pattern of false rate drop detections and pacing intervention will continue. This cyclical pattern will be disrupted only when intrinsic heart activity is sensed above the lower rate, thereby inhibiting pacing output and preventing another detection of a rate drop according to the lower rate criterion. If the patient's intrinsic rate is lower than the programmed lower rate, for example while sleeping, repetitive re-triggering of the rate drop response can occur. Such repetitive intervention is unneeded and undesirable since pacemaker battery energy is unnecessarily consumed and the patient's sleep may be disrupted. For patients that do not experience rate drop episodes at night, a sleep disable feature can prevent false rate drop detections at night as disclosed in the above cited '525 patent to Shelton. However, in some patients, the lower rate criterion may be needed to ensure that unpredictable rate drop episodes are detected night or day.
A need remains, therefore, for providing a cardiac pacing device and method for detecting and treating a heart rate drop, associated with vasovagal syncope, or other vasodepressive or cardioinhibitory disorders such as carotid sinus syndrome, that avoids repetitive rate drop response interventions triggered by a lower rate criterion enabled to detect unpredictable rate drop episodes.